Defender's Quest. Fun writing and addictive gameplay. Disclaimer: I don't normally play tower defence.

Oh, yeah. I guess the "creeper world" series can qualify as tower defense. In a way.

In this case, it blows all the others out of the water. If I may use this metaphor here.

No, it's not. Onnce you understand certain tactics and know how to use them, it's not even hard to beat the last levels on NG+ with just the original six characters.

I definitely recommend Defender's Quest. Don't even like tower defense games, but I had such a good time with that one (and still play it from time to time).

This, I personally preferred GemCraft Labyrinth of the three since it gives you the chance to alter the paths.
GemCraft Labyrinth.

It can become repetitive at times but I always came back to it.

Like so many said: Defender's Quest.
And if you don't mind playing in a browser: Cursed Treasure 1 and 2, Creeper world, Bubble tanks, Bloons series.

Well, only 0.9 percent or less COMPLETED it on Steam, so I assume we have different definitions of "completed".

For me on a commercial achievement game it means 100p. On Kongregate it means for me "get the Kongregate badges", regardless of ingame achievements.

If you go for finishing the main story but not going for achievements, then yes, Defender Quest is a nice one but IMO nothing too special. Worth to get it on a sale, but not for the full price.

You are correct. Completing a game is finishing the main and all side quests for me. Not grinding every meaningless achievement. Also on steam people just start the game to farm trading cards, not even playing one mission. Stats mean shit on steam.